Various conversion coatings on metal surfaces have been suggested in the prior art for the purpose of forming a coating which protects the metal against corrosion and also serves as a base for improving the adhesion of subsequently applied siccative organic finishes. Such conversion coatings are applied by treatment of the surfaces with solutions of various chemicals which react with the surface to form the desired coating. Among the commonly used conversion coating compositions are aqueous phosphate and chromate solutions. Among the simplest of the phosphate compositions are the so-called iron phosphates which comprise, for example, solutions of alkali metal phosphates, and which react with the iron on the metal surface to form an iron phosphate coating.
It has long been known that surfaces of zinc and zinc-based alloys can be protected against corrosion by treatment with an acid solution containing hexavalent chromium. It was suggested that the attack of the solution on the surface is facilitated if the solution initially contained a small amount of trivalent chromium, and it has been proposed to introduce this trivalent chromium by adding a compound of trivalent chromium or preferably by adding a small amount of a suitable reducing agent. As the solutions are used, more trivalent chromium is formed by reduction of hexavalent chromium at the zinc surfaces so that the concentration of trivalent chromium progressively increases and the solution eventually has to be discarded when the quality of the coating is affected by the deterioration of the solution. Examples of patents which describe solutions containing mixtures of trivalent chromium and hexavalent chromium include U.S. Pat. Nos. 3,880,772; 3,795,549; 3,553,034; 3,404,046; 3,090,710; 2,911,332; and 2,902,392. The treatment of zinc surfaces with solutions wherein chromium is entirely in a trivalent state is disclosed in U.S. Pat. Nos. 4,171,231; 3,932,198; 3,647,569; 3,501,352; and 2,559,878. Trivalent chromium solutions are also disclosed in British Patent 1,461,244.
The prior art chromium solutions and the coatings obtained therefrom have not been entirely satisfactory in that they have not always been able to meet the requirements of the zinc plating industry. One of the important requirements of the chromium-containing solution is the ability of the solution to impart a clear to light blue finish on the zinc or zinc alloy surface. In the past, the achievement of this finish on zinc plated from a cyanide containing solution was a relatively easy process using conventional chromates containing hexavalent chromium compounds sometimes in combination with other species such as nitrates, fluorides, sulfates, etc. However, with the advent of alkaline non-cyanide type zinc plating solutions, the production of the proper finish of the zinc after chromating has been a difficult procedure. The difficulties generally have been thought to result from the co-deposition and inclusion of relatively large amounts of organic materials from the zinc brightener components.
Another problem area in chromating is that of iron contamination of the plating bath which causes black staining of the zinc plate when chromated. This problem frequently is encountered in alkaline non-cyanide baths that have been converted from cyanide containing baths. Cyanide baths generally contain relatively large amounts of complexed iron in the form of ferrocyanides. When the free cyanide concentration of the bath reaches approximately zero, these ferrocyanides begin to decompose during electrolysis, and iron is codeposited in the zinc deposit. This problem also can occur in non-cyanide zinc baths which utilize strong chelating or complexing agents as part of the additive system. The source of iron in this case generally is from drag-in of dissolved iron from the preceding acid pickling tanks. The iron staining problem is most frequently encountered in acid zinc plating baths where the pH is low enough that iron can be dissolved in the bath either from non-plated areas of the parts being processed or from parts laying in the bottom of the plating tank. The generally accepted explanation for the black staining is that the hexavalent chromium compounds used in conventional chromates react with codeposited iron to form black iron oxides.
Another disadvantage of hexavalent chromium type solutions is in the area of waste disposal. Recent emphasis on water pollution problems has drawn attention to the fact that chromates are serious pollutants. In order to satisfy water quality standards, it frequently is necessary to subject the waste water to a multi-stage purification sequence in order to remove chromates from the effluents. Typical steps in the sequence include the reduction of any hexavalent chromium to trivalent chromium and precipitation with, for example, lime. This precipitation results in a reduction in the chromate content of the effluent water but the process is quite expensive. Another problem which has been observed with chromate finishes which have been described previously is the unacceptable adhesion characteristics when certain paints have been applied over the chromate coatings, particularly on exposure to salt spray.
Non-chromate conversion coating solutions are disclosed, for example, in U.S. Pat. Nos. 4,225,351; 4,225,350 and 4,222,779 for the treatment of zinc, cadmium, silver, copper, aluminum, magnesium and zinc alloys. Each of these patents discloses a bath constituted of an aqueous solution of from about 0.2 to about 45 grams per liter of free H.sub.2 SO.sub.4, from about 1.5 to about 58 grams per liter of H.sub.2 O.sub.2, from about 3 to about 33 grams per liter of SiO.sub.2 and from about 0.15 to about 10 grams per liter of an organophosphorus promoter. In each of these patents, the preferred organophosphorus promoter is 1-hydroxyethylidene-1,1-diphosphonic acid. U.S. Pat. No. 4,225,351 provides for the addition of a secondary promoter selected from the group consisting of ascorbic acid, boric acid, gluconic acid, glycolic acid, tartaric acid and the salts of these acids to the foregoing solution. U.S. Pat. No. 4,225,350 discloses the addition of a cationic triarylmethane dye to this solution. A disadvantage of using the non-chromate solutions disclosed in the foregoing patents is the tendency of these solutions to gel after a relatively shortened period. This tendency to gel not only limits the useful life of the bath but also prohibits the effective use of providing concentrates for starting or maintaining baths of these solutions.
It would be advantageous to provide a non-chromate conversion coating for use with zinc, zinc alloy and cadmium surfaces from a solution that was stable over a relatively indefinite period of time and could be started and maintained with concentrates.